A touch and contour sensing device, and more specifically, a device that senses user inputs either with a finger or stylus, or images textured surfaces like fingerprints is vulnerable to damage, caused by excessive mechanical forces during usage. Irreversible damage can also be caused by electrostatic discharge or ESD, since user objects or the user's finger may carry significant potential. To protect such a sensing device from mechanical abrasion, impact or scratching, and to also protect the sensing surface from electrostatic discharge induced damage, specialized and expensive techniques are known to cover the device with a protective coating.
In U.S. Pat. No. 6,693,441, Lane et al. describe a protective film coating made of a polymer containing conductive particles. In their preferred embodiment, the coating is intended as protection against mechanical impact. The coating contains conductive particles, and it is sprayed on to the sensing surface. One of the drawbacks is that the protective coating is only made of Teflon, which is a soft material, and can be scratched with sharp objects. In addition, the carbon particles that serve as conductive particles within the Teflon polymer matrix are also soft, and thus do not enable effective protection against mechanical damage. Another drawback is that the conductive particles in the coating are insulated from each other within the polymer, as a result of which the coating itself is not conductive. Since the coating as a whole is not conductive, it cannot provide protection against electrostatic discharge.
In U.S. Pat. No. 6,346,739, Lepert et al. describe a method of and apparatus for dissipation of electrostatic discharge using a patterned layer of conductive material that is deposited by vacuum deposition, and is an integrally formed part of the sensor to ensure that the gap between conductive plates is left uncovered. A separate insulating sheet film is then applied over this patterned conductive material layer, and electrical connections to the top surface are required to provide an electrical path.
Reference is hereby made to U.S. patent application Ser. No. 10/038,505 filed Dec. 20, 2001, titled “Fingerprint Image Sensing Device With Passive Sensor Array.” Reference is also made to U.S. Pat. No. 6,578,436 titled “Method and Apparatus for Pressure Sensing.” These references describe the use of a single “sheet film coating” made of a polymer such as MYLAR®, which provides limited mechanical protection as well as transmitting the force of the pressure applied, to the sensing elements on the sensing device by virtue of the effect of force concentration.
One drawback, however, of the sheet film protective coating in use such as the FID-102 above is that the coating, upon application of the occasional, excessive, accidental force like pressure from the fingernail of a finger, pressure from the tip of a pen, or the rubbing of an abrasive material on it, can get deformed or permanently indented. Once this happens, the images obtained from the sensing device are likely to be inaccurate or distorted. On the other hand, a thicker coating may be used so that it is less vulnerable to deforming forces. However, increasing the thickness makes the coating progressively mechanically stiffer, thereby leading to a loss of resolution on sensors that require high resolution, such as the fingerprint image sensor. A thick or inflexible coating can lead to cross-talk between adjacent switches on devices such as the fingerprint image sensor, as switches disposed under valleys on a fingerprint of a finger are pressed downward by the downward motion of the coating pressed down by adjacent ridges.
Accordingly, the present inventors have determined that there is a need for a protective covering that has various properties and can be applied without expensive and difficult techniques. One property is the coating being resistant to oil, water, alcohol, moisture, grease, and such chemical agents. Another is being resistant to mechanical damage caused by abrasive impact, scratches, and wear. Another property is being flexible enough to withstand the impact of an occasional, accidental, potentially mechanically damaging force, like that of a fingernail, pen tip, or abrasive material, as mentioned earlier. Still another is being sensitive enough so that there is no loss in resolution on the sensing device. And another property is having conductive properties, either inherent or induced, so that it can conduct electrostatic discharge away from the underlying components of the sensing device. Different embodiments of the present invention meet some or all the aforementioned properties.